"Additional Resources" feature for chapters

We are creating a plugin for that. Creating fields for links. And the citiations of the chapter and more.

In order to be able to us the plugin and to extend the number of links or information, we think it would be good to have on the book theme a new buttom in the sidebar.

Instead of Resources, as in the picture, can be named as More, for a more neutral name. In that way we have a neutral space for extending content as we need it.